(1) Field of the Invention
This invention relates to hydraulic pressure producing systems and more particularly to a hydraulic pressure producing system suitable for driving a press and the like.
(2) Description of the Prior Art
A hydraulic pressure producing system for a press generally operates such that the pump is directly driven by the motor as the latter rotates, to drive the press or other load (the piston of a hydraulic cylinder, to be exact) by the discharge pressure of the pump.
The hydraulic pressure producing system of the type described includes two types: one type has an oil tank located separately from the pump for supplying liquid in circulation flow through a line to the pump to obtain the discharge pressure of the pump, and the other type has the pump itself immersed in the liquid and driven while being submerged in the liquid so that the discharge pressure of the pump can be taken out to outside. Of these two types, the latter has in recent years come to attract attention because the system of this type is simple in construction, compact in size and light in weight. However, some disadvantages are associated with these two types of hydraulic pressure producing system of the prior art. That is, as described in Japanese Pat. No. 44881/73, they operate at a high energy consumption level because the pump rotates at all times regardless of whether the pump is at load or no load, noises are produced at all times because the pump rotates at all times, and a large amount of heat is generated by the rotating pump. The noises produced by the pump can be somewhat reduced in the system of the latter type in which the pump itself is submerged in the liquid, but it is impossible to avoid noise production entirely when the pump is at no load. However, since the pump is responsible for the majority of noises produced by the hydraulic pressure producing system, no satisfactory noise characteristics could be produced by using this type alone. For example, let us observe noises produced by internal-gear pumps of the former type of varying capacities or pumps of 64 cc/rev.[I], 100 cc/rev.[II] and 125 cc/rev.[III]. As shown in FIG. 9, when the rpm (N) of the pumps is 1500, the noises produced by the pump [III] of the highest capacity reach a very high level or over 80 phon if the discharge pressure exceeds 200 kg/cm.sup.2. When a press or other load is connected to the hydraulic pressure producing system, unnecessarily large noises would be produced during the time the press is driven which accounts for about 50% of the total press operation time. Also, it will be seen from FIG. 8 that no-load time zone B exists at all times between the loaded time zones A (in which the press is driven) so that not only the noises but also a loss of energy is high in level.
From the point of view of performing the operation in safety, the hydraulic pressure producing system of the prior art has disadvantages. For example, almost all the presses are controlled by a hydraulic valve in their operation. When the control valve for controlling the discharge pressure of the pump fails, it is impossible to render the pump inoperative unless the motor is disconnected from the power source. Thus the hydraulic pressure producing system of the prior art does not lend itself to quick starting and quick shut-down. Since difficulties are encountered in rendering the pump inoperative quickly when trouble arises, the hydraulic pressure producing system of the prior art has been unable to operate with assured safety.